1. Field
One or more exemplary embodiments relate to a system and method of controlling a compressor, and more particularly, to a system and method of controlling a compressor, by which influences of an inlet guide vane and an anti-surge valve on each other are reduced and a compressor may stably operate.
2. Description of the Related Art
A fluid control system of controlling liquid or fluid in a gaseous state uses a compressor for compressing a fluid. A compressor is designed to operate with a high efficiency with respect to ejection pressure and flow rate in a range as wide as possible. Not only the efficiency of a compressor, but also an operating range of a compressor may function as important performance variables of the fluid control system.
For example, in a turbo compressor, when the turbo compressor fails to produce a pressure greater than a pressure resistance of an entire fluid control system, a backflow phenomenon of a cyclic flow in the compressor is generated, which is referred to as “surge”.
When a surge phenomenon occurs, a pressure and a flow rate are perturbed by the cyclic backflow of flow. The perturbation generates mechanical vibration and damages accessory elements such as a bearing or an impeller. As such, the surge phenomenon may not only deteriorate the performance of a compressor but also shorten the lifespan of a compressor. Accordingly, in the operation of a compressor, a function of preventing a surge phenomenon (anti-surge) is important in a compressor control system for controlling a turbo compressor. When an anti-surge valve (ASV) is used to implement the anti-surge function, the generation of a surge phenomenon may be prevented by reducing the resistance of a fluid system.
Also, in the compressor control system, in addition to the anti-surge valve, an inlet guide vane (IGV) is installed at an inlet of a compressor to control the operating range of the compressor.
FIG. 1 is a performance map of a general compressor. In FIG. 1, the vertical axis denotes pressure and the horizontal axis denotes a flow rate. Surge control is performed by setting a surge control line with a margin of about 10% from a surge line and controlling an IGV or ASV when an operating point reaches the surge control line, so that the operating point is moved away from the surge line.
Since the ASV does not operate until the operating point reaches the surge line, a coupling phenomenon is not generated. However, when the operating point enters a surge area in which the surge phenomenon may occur, both the IGV and the ASV are operated. Since both the IGV and the ASV change the pressure and flow rate of a compressor, the coupling phenomenon may occur as the IGV and the ASV are operated together.
FIG. 2 is a graph for explaining a coupling phenomenon occurring in the IGV and the ASV of the compressor of FIG. 1.
For example, assuming that a compressor is controlled at an operating point A in FIG. 2, a direction to control the IGV and a direction to control the ASV may interfere with each other. In other words, to decrease pressure, the IGV is controlled in a direction to decrease (shut) a degree of opening (opening range) of the IGV. When the IGV is controlled to decrease the opening range, a flow rate and pressure are decreased and thus the control point of IGV in FIG. 2 is moved in a direction toward the lower left corner.
However, to increase the flow rate, the ASV is controlled in a direction to increase an opening range of the ASV, thereby preventing a surge phenomenon. When the ASV is controlled in a direction to open the ASV, the flow rate increases and the pressure decreases and the control point of ASV in FIG. 2 is moved in a direction toward the lower right corner. As such, since interference occurs between the control operations of the IGV and the ASV, a pressure hunting phenomenon occurs so that an unstable flow is repeated and thus the operation of a compressor becomes unstable.
The coupling phenomenon is generated because although the ejection pressure of a compressor is controlled by the operation of IGV, the flow rate is affected by the operation of IGV, and although the control using the flow rate of a compressor is possible by the operation of ASV, the pressure is affected by the operation of ASV. Accordingly, in the surge area, two valves of the IGV and the ASV interfere with mutual operations so that it is difficult to control the overall system of a compressor.
To avoid the interference problem between the control operations of the ASV and the IGV as described above, a method of reducing interference in the surge area is used in which a control gain of a proportional-integral-derivative (PID) controller for the control of the IGV and the ASV is set to be different from each other and the control gain of any one of the IGV and the ASV is set to be dominant. However, the gain control method has a limit in that tuning of a controller is complicated and difficult, and the coupling phenomenon is not completely addressed.
For example, when an IGV gain is set to be relatively greater than an ASV gain, although the pressure is stabilized, as the operating point enters the surge area, the surge phenomenon is highly likely to occur or it is difficult to deal with a sudden change in the consumption flow rate at the rear end of a compressor.
Also, when the ASV gain is set to be relatively greater than the IGV gain, as the ASV is quickly opened when the operating point enters the surge area, the range of pressure drop increases. In this case, although a sudden change in the consumption flow rate may be coped with, as the range of pressure drop increases, the operation of a compressor may be unstable.